1. Field of the Invention
The present invention relates to a temperature controller of a liquid cooling system for maintaining the temperature of coolant in a machine, such as a machine tool, at a predetermined value using a refrigeration circuit and, more particularly, to an improvement in a control precision of the coolant temperature in consideration of the change of an operating condition, or a parameter relevant to the quantity of heat generated in a machine.
2. Description of a Prior Art
Conventionally, as a temperature controller of a liquid cooling system for controlling a coolant temperature in a machine, such as a machine tool, at a predetermined value, a system has been well known in which coolant is cooled using a refrigeration circuit to hold the coolant temperature constant.
For example, as disclosed by the Japanese Utility Model Publication Gazette No. 48-27351, a temperature grade around a machine such as a machine tool is held constant in the course of operation and the heat strain of each part of the machine is restrained to the extent possible by measuring the temperature of cooling oil in the machine and the temperature of the machine, and feedback-controlling the temperature difference between the cooling oil temperature and the machine temperature to maintain it at a previously set value.
In addition, as disclosed by the Japanese Patent Publication Gazette No. 46-16216, it is known that the same effect as mentioned above can also be obtained by maintaining the temperature difference between the cooling oil temperature and the room temperature at a predetermined set value.
In both cases, in order to maintain the temperature difference at a set value, a compressor in a refrigeration circuit is turned ON and OFF in accordance with the difference between the temperature difference and the target set value for control. More specifically, as shown in FIG. 12, when the temperature difference reaches the set value from the high temperature side, the compressor is stopped (a1, a2, . . . ). On the other hand, when the temperature difference rises from the set value by a certain quantity, the compressor starts its operation (b1, b2, . . . ). Therefore, the cooling oil temperature changes considerably in accordance with the change of ON and OFF of the compressor as shown by a solid line in FIG. 12 and the change of the machine temperature in accordance with the above-mentioned change is delayed, so that the hunting of the control could be generated.
As disclosed by the Japanese Patent Laying Open Gazette No. 60-131149 or the Japanese Patent Laying Open Gazette No. 60-133273, it is considered that the change of the coolant temperature is smoothed and a temperature control precision of a machine can be improved by adjusting the operating capacity of a compressor in a refrigeration circuit having an inverter with a variable frequency.
However, when the operating condition of the machine changes, for example, when the rotation frequency of a main shaft of the machine tool increases while the coolant temperature is feedback-controlled, it takes considerable time until the machine temperature, that is, the coolant temperature changes by the above-mentioned change. As a result, hunting due to the response delay of the control could be generated for example, so that there is the possibility that a stable control can not be performed.